Nobody seems to be answering this question, so I'll make a start.
Soays are slow growing, so will only really reach a suitable size when they are in their second year (ie hogget rather than lamb). It's unlikely they will ever get fat. This means that as commercial sheep grown for meat, they are just not suitable, and your comment that they may not get a bid at market is potentially true (except at specialist rare breed sales).
On the other hand, they are lovely small sheep, which can become very tame and yet alert enough to keep you on your toes. They lamb easily, without human intervention (generally) and seem to have less troubles than many other sheep. When they are ready for meat at about 15 months, they taste beautiful - real gourmet meat that will sell to people that appreciate them. On the other hand, they are half the size of commercial sheep & have taken twice as long to get there, so they can never compete with cheap lamb.
Basically, you keep Soays because you like them, not to make a profit. Choose commercial sheep such as Beltex, Texel or Suffolk if you want to make money. Perhaps this is why Soays are a rare breed sheep? They do give a lot of pleasure though, and I enjoy keeping them.
John